This invention relates to target materials for sputter coating sources in general, and more specifically to specially designed magnetron sources in which a "closed circuit" magnetic path is used to generate a fringing magnetic field over the surface of a non-magnetic target.
Sputtering is a process that falls in the general class of vacuum coating processes. These processes are used to deposit a thin layer of desirable material on the surface or surfaces of another object to provide a particular function or to enhance the appearance of the object. As an example, thin films of aluminum or of aluminum alloys, of the order of one micron in thickness, are applied to silicon wafers in the production of integrated circuits to form the electrical connections between individual semiconductor devices. As another example, thin films of aluminum, chrome, brass, and other metals are applied to the surfaces of plastic objects, such as automotive headlamp bezels, door handles, and door lock plungers to impart to the objects a pleasing metallic appearance.
The sputtering process is a plasma process in which positive ions of an inert gas impinge upon the surface of a target material made up of the material desired for the film. As an example, in a sputtering process of coating a plastic object with chromium, the target material would be the chromium. Sputtering is accomplished from a device called a sputter coating source. Such a source embodies an electrical system for biasing a target material structure with a negative voltage, either DC for electrically conductive targets, or RF for non-conductive targets, so the target will attract positive ions from a plasma of an inert gas that is established in the region of the target. The sputter coating source also contains a system for cooling the target structure and often a magnetic structure for containing and enhancing the plasma.
Positive ions from the plasma are extracted and accelerated to a high kinetic energy to strike the surface of the target structure, where part of the kinetic energy is degraded to heat, and part is imparted by momentum transfer to atoms of the target material. Such atoms that gain sufficient energy to overcome their bonding energy escape from the target surface and are ejected into the vacuum chamber that houses the process. Objects placed in line-of-sight of an operating sputter coating source are coated by the atoms ejected from the target surface.
There are many United States and foreign patents relating to the design and fabrication of both sputter coating sources and sputtering targets for use in such sources. Representative examples include: U.S. Pat. No. 4,166,018, entitled "Sputtering Process and Apparatus", by John S. Chapin; U.S. Pat. No. 4,401,539, "Sputtering Cathode Structure for Sputtering Apparatuses, Method of Controlling Magnetic Flux Generated by Said Sputtering Cathode Structure, and Method of Forming Films by Use of Said sputtering Cathode Structure", by Katsue Abe et al.; and U.S. Pat. No. 4,414,086, "Magnetic Targets for Use in Sputter Coating Apparatus", by Lawrence T. Lamont, Jr.
Sputtering sources known as "diode" sources were the first to be built, and did not use magnetic fields. Commercial application of sputter coating sources has been greatly enhanced by the use of magnetic fields to trap electrons and confine the plasma close to the sputtering target surface, which also enhances the plasma energy and the resulting rate at which the material may be sputtered. U.S. Pat. No. 4,166,018 by John S. Chapin is a typical example of a magnetron sputter coating source. The word "magnetron" source has come to be used by those familiar with the art of sputtering to signify the use of magnetic fields to confine and enhance the plasma in the sputtering process.
This prior art is best understood in the context of simple physical structures that embody the general principles used in the art. By way of illustration, FIG. 1 shows a typical prior art flat (planar) non-magnetic rectangular sputtering target 11 (also called a sputtering cathode) of length "L", width "W", and thickness "T1", with a magnetic field imposed though the target and forming a closed magnetic tunnel. The magnetic field (B-Field) is represented by flux lines 12 and an imaginary plane 13 is shown passing the width of the target. For the purpose of this illustration, the target is assumed to be of non-magnetically permeable material, and therefore transparent to magnetic fields. Similarly, FIG. 2A shows a typical flat disc, non-magnetic, sputtering target 21 with a diameter "D" and thickness "T2", also having a magnetic field imposed through the target to form a closed tunnel. The magnetic field is represented by lines 22 and an imaginary plane 23 is shown bisecting the target through the center of the target disc. FIG. 2B shows a cross-sectional view through the imaginary plane 23 of FIG. 2A and illustrates the spacial relationship between the target and magnet structure 25 used to create the magnetic field through the target. In this case, magnet structure 25 is shown with its poles directly behind the target. Magnetic lines of force 22 extend from one pole to the other of the magnet structure, and, because of the assumption that the target is non-magnetic, the lines of force are not effected by the target material. The magnetic poles are shown to be placed a short distance "x" behind the target, although they could in theory be in contact therewith. The magnets may be either permanent magnets, in which case the strength of the field will be constant, or they may be electromagnets, as illustrated in the figure, by applying voltage "V" to coils 26 to generate a current "I" in the windings. A clear advantage in using electromagnets is the fact that as a target erodes due to the sputtering process, the strength of the magnetic field may be adjusted to maintain a constant plasma impedance. This can be done by adjusting the coil current "I" by changing the applied coil voltage "V". In this illustration of magnetron source principles, some of the required characteristics to form and maintain a plasma are not shown, such as an applied negative voltage on the target, and a vacuum enclosure for the process.
As illustrated in FIG. 2B, the magnetic field lines 22 are more concentrated at the inner poles (S) than at the outer poles (N). This polarity is a matter of convention and the situation would be the same if the inner poles were (N) and the outer poles were (S). There is also a skewing of the field toward the outer poles, the reason for which will become apparent shortly.
FIG. 2C, a top view of the disc-shaped cathode of FIGS. 2A and 2B, shows that the inner pole structure, indicated by shaded area 4, is much smaller in area than the outer pole structure, indicated by the shaded area 3. Hence, the magnetic field created between the poles will have greater density (more field lines per unit area) at the smaller area inner pole than at the larger area outer pole. This effect of unequal areas causes skewing of the magnetic lines of force to the outside of the disc as shown schemmatically in FIG. 2B. This skewing of the B-Field will cause asymmetrical erosion of the target surface illustrated by dotted lines 27 and 28 in FIG. 2B. Similar effects are seen at the ends of the rectangular planar target illustrated in FIG. 1, and in other configurations where the magnetic structure is curved. This skewed, lower density, magnetic field to the outside can cause erosion of support and clamping structures and overheating of ground shielding that may be positioned around the target. It can also provide a relatively easy escape path for electrons to the ground shielding, anodes, or the chamber structure, thereby lowering the plasma impedance and, in turn, the operating voltage, and can result in other operating problems as well. In such cases, the plasma will not be well confined around the outside of the magnetron sputter coating source, an effect commonly called "blooming". Another, detrimental phenomemon, the magnetic mirror effect, makes the blooming of the plasma to the outside even more pronounced due to the unequal areas of the pole structures of curved sources.
To appreciate the significance of the above described problems, it is best to first discuss some of the basic physics of the sputtering process, which can be understood by referring to FIG. 2D. Target 21 is shown connected to a power supply 29 at the negative terminal so that the target is biased at a negative electrical potential relative to the surrounding structures. The positive terminal of power supply 29 is connected to earth ground, as are the walls (not shown) of the vacuum enclosure and possibly other structures, often called anodes, surrounding the target. This power supply causes an electrical force field, hereinafter the E-Field, to be established between the target surface and surrounding structures, represented in FIG. 2D by lines of force E. The arrangement of components in a magnetron sputter coating source is such that the E-Field is generally perpendicular to the B-Field.
Before a magnetron sputter coating souce is ignited, most of the air is pumped away from the target enclosure. Next, an inert gas such as argon is introduced at low pressure (.about.10.sup.-2 Torr) into the enclosure surrounding the sputter coating source. This gas is represented by a single neutral argon atom 30, although there will actually be very many neutral argon atoms in the enclosure.
When the power supply 29 is turned on, an electrical potential is established from the target surface to the chamber walls and/or anodes corresponding to the E-Field, E, and electrons are emitted from the target surface. In the absence of an electrically conductive medium, relatively few electrons will be emitted, and only a very small electrical current will flow between the target surface and the chamber walls and/or anode structures. An electron 2, traveling in the presence of a magnetic field will tend to be "captured" and will follow a spiral path around a line of magnetic force, such as line B. If the electron has an initial velocity, V, which has a component along the line of magnetic force, the electron will travel along the line until it is repelled from the target surface by the electric field indicated by vectors E8 and E9 (since both the target surface and the electron have negative electrical charge, and like charges repel). Therefore, electrons emitted from the target surface will not return to the surface, but the effect of the magnetic field will be to retain such electrons near the surface. As a result of this repulsion, however, the electrons do tend to transfer to magnetic field lines which extend farther from the target surface as the electrons execute their spiral path, and hence they eventually wander away from the immediate surface of the target.
Another force acting on electron 2 is a result of the crossed B-Field and E-Field. In the presence of crossed fields, the electron is acted upon by a force perpendicular to both the E-Field and the B-Field, tending to impart to the electron a "drift" velocity in the direction of the vector "D" in FIG. 2D. This drift velocity tends to carry the electron along the direction of the magnetic tunnel illustrated in FIG. 2A. The net result then of the repulsion and the crossed E-Field and B-Field is that the electron will tend to travel gradually away from the target surface, while at the same time drifting along the direction of the tunnel. For this reason, the magnetic tunnels are typically designed to close on themselves, in order that the electrons will not be lost from the vicinity of the target surface due to this drift effect.
The magnetic mirror effect can be explained by referring to FIG. 2E, which shows a portion of the cross-section through plane 23 of the round planar magnetron target 21 of FIG. 2A. The B-Field pattern is illustrated by lines of force B1 through B5, which again are shown as skewed due to the unequal areas of the pole pieces 25N and 25S. This skewing of the field results in the lines of force converging toward the inner pole pieces and diverging toward the outside and away from the outer pole pieces. This converging of the magnetic field at the inner pole pieces means that the field strength increases there, since there are more lines per unit area in that region. Because of this increase in field strength, as an electron travels along a line of force, it spirals in an ever tighter orbit, converting more and more translational energy into energy of rotation, until finally the velocity along the field line vanishes. Then the electron turns around, still spiraling in the same direction, and moves backward substantially from whence it came. In a sense, it has been reflected, and hence the name the magnetic mirror effect. This effect can be thought of as a force field, represented by vectors F1 through F5, which tend to push electrons along the lines of force away from the area of convergence of the field lines.
The net effect of these forces, that of repulsion from the target surface, the magnetic mirror effect due to the converging magnetic field, and the drift force due to the crossed magnetic fields, is to cause electrons to drift around the magnetic tunnel, but also to be forced upward and away from the center of curvature of the magnetic structure into the area to the outside where there is lesser field intensity. These same effects occur at the ends of a rectangular planar magnetron and for other curved magnetic structures.
These forces have important implications for the sputtering process, for it is the electrons trapped near the surface of the target that drive the process. The trapped electrons collide with neutral argon atoms 30, and in a percentage of such collisions an electron is stripped from the argon atom, adding to the electron population and creating positively charged argon ions 9. Ions thus created are strongly attracted to the target surface because of the negative electrical potential maintained on the target. As these ions impact the target surface, some of the kinetic energy is converted to heat, and in a certain percentage of these collisons, neutral atoms of the target material (represented by metal atom 8 in FIG. 2D) gain sufficient energy by momentum transfer to escape from the target surface and are ejected into the process volume. Such neutral target atoms are not effected by the electrical and magnetic fields, and impact any object surface in line-of-sight of the target surface, creating a coating of the target material on the object surface.
The desired effect, then, is to trap electrons as efficiently as possible to cause efficient ionization of the inert gas. The blooming effect to the outside of curved planar sources, away from the center of curvature, is detrimental because it allows electrons to escape more easily to the outside, and can result in overheating and erosion damage to surrounding structures.
In all of the background discussed thus far, the assumption has been made that the target material is non-magnetically permeable, such as aluminum or chromium, and the targets are therefore transparent to magnetic fields. However, magnetic targets are an important element of the art. FIG. 3 shows a cross-section of a prior art, disc-shaped, planar target similar to FIG. 2A, except that the target material is now assumed to be magnetically permeable, and the poles of the magnet structure are in contact with the back of the target. In this case, there are considerable differences in the way the magnetic circuit operates to create the necessary magnetic tunnel on the target surface.
In the non-magnetic case, poles of opposite polarity are created at the ends of the magnet structure 25 in FIG. 2B and a magnetic field is established with lines of force extending from these poles, labeled N and S. With a magnetic target, as shown in FIG. 3, the magnetic flux, indicated by flux lines 7, will tend to pass through the magnetic target radially and return to the opposite pole of the magnet structure, so that a "closed" magnetic circuit is created. If the entire closed path is of sufficient cross-sectional area and relatively low values of magnetomotive force are applied, most of the magnetic flux created will be confined to the target and the magnet structure, and there will be very little fringing field. As the magnetomotive force is increased, however, a point will be reached where some portion of the path becomes magnetically saturated, i.e., the value of flux (lines of force) per unit area is reached above which the internal flux density cannot increase. Additional flux created by increasing the magnetomotive force can only be accomodated in the circuit by fringing into the volume surrounding the saturated portion, in effect by-passing the region of saturation.
By design in such an arrangement, the magnet structure is made of sufficient cross-section that saturation will take place in the target material before it occurs in the magnet structure. For disc-shaped targets, this saturation occurs very near the inner pole, because this is the point in the flux path in the target of the least cross-sectional area. In FIG. 3 this region of saturation is shown at 38, at a radius "r" from the center of curvature, just to the outside of the contact area of the inner pole of the magnet structure 33. Essentially, the portion 39 of the magnetic target to the inside (toward the center of curvature), of the radius of saturation becomes an extension of the inner pole, and the portion 37 of the magnetic target to the outside of the radius of saturation becomes an extension of the outer pole of the magnet structure. The magnetic poles are now immediately adjacent to one another on the target surface instead of at the ends of the magnet structure 33, and the fringing field 32 created is across an apparently much smaller gap than is the case for the non-magnetic target illustrated in FIG. 2B.
There are two quite beneficial effects in this latter arrangement. One is that the smaller gap requires less magnetomotive force to create a sufficient fringing field (magnetic tunnel) for the operation of a magnetron sputter coating source. The other is that the effect of the outer magnetic pole, which places the outer pole very near the inner pole, separated only by the region of saturation, serves to confine the magnetic field more effectively than is the case with non-magnetic targets. Hence, the "blooming" effect of the plasma is greatly reduced. The outer periphery of the magnetic field is nearly as dense as the field near the region of saturation, and the magnetic mirror effect is not so evident. As a result, electron escape to the outside is reduced, and the position and control of the magnetic tunnel and the operating characteristics of the source are improved.
Given the dramatic improvements evident in magnetic target systems, what is needed is a magnetic enhancement system for non-magnetic target structures which is as effective as magnetic target systems in containing electrons near the target surface and in maintaining position and control over the magnetic tunnel.